Radar systems are critical to the operations of various agencies such as the National Oceanic and Atmospheric Administration (NOAA), National Weather Service (NWS), the Federal Aviation Administration (FAA), the Department of Defense (DOD) and the Department of Homeland Security (DHS). These agencies have numerous varying requirements and potential applications for modern radar technology. These applications include weather observation, meteorological research, aircraft tracking, unmanned aircraft systems surveillance and surface transportation. As each of these agencies requires a radar system with unique capabilities, the current radar infrastructure comprises a wide variety of radar types and configurations. For example, weather tracking operations performed by the NOAA and FAA may require a radar system capable of dual-polarization (dual-pol) operation. However, radar tasks associated with the DHS and/or other FAA operations do not have such requirements. Likewise, a narrow beamwidth, for example, no greater than one (1) degree, may be required for certain tracking operations but not for others. These parameters may necessitate variations in antenna array size, radiating element type, cost, and/or complexity.
As many existing radar systems are coming due for replacement, government and industry are working to transition from these individualized systems to a universal system capable of meeting the various agency demands, while offering reduced complexity and substantial cost savings. One proposed solution is digital phased array technology, specifically multi-function phased array radar (MPAR). A phased array antenna is comprised of numerous radiating elements each having a phase shifter. Beams are formed by selectively activating all or a portion of antenna elements of a given array. Scanning or steering of the beams is accomplished by shifting the phase of the signals emitted from the elements in order to provide constructive and/or destructive interference. The ability to form and steer a radar beam permits multiple functions to be performed by the same radar system. In addition to multi-function operation, these arrays have a quicker response time and operate at a higher resolution than existing rotating radar systems.
While MPAR offers advantages over current systems, several technical and economic obstacles need to be overcome before it can be implemented successfully as a universal system. One example of an application creating technical challenges is weather-related radar systems. Particularly, singularly-polarized (single-pol) systems (systems transmitting a signal oriented on one axis, e.g. a horizontal axis) are largely incapable of hydrometeor classification, as rain drops, water droplets, vapor and ice particles each have different geometric shapes and physical properties when falling. Single-pol beams measure precipitation as if it consists of only perfect spheres, and therefore cannot effectively estimate the size of the precipitation, and thus the type and amount thereof.
To overcome this shortcoming, radar systems have been developed which transmit alternating or simultaneous pulses of horizontally and vertically polarized signals using arrays possessing orthogonally oriented radiating antenna elements. These dual-pol radar systems, or “polarimetric” systems, offer several advantages over conventional single-pol radars in estimating precipitation types and amounts. For instance, by measuring along two axes, these systems have the capability of discriminating between hail and rain, estimating rainfall volume and detecting mixed precipitation. Accordingly, most weather radars, such as those operated under NOAA requirements, as well as the FAA to monitor potential plane icing conditions, utilize dual-pol radar arrays.
A number of current dual-pol radar systems transmit sequential pulses of differing polarizations (i.e. a horizontal pulse followed by a vertical pulse). However, these spaced pulses reduce the effective velocity range of the radar, in addition to creating long dwell times. Specifically, in order to correlate the data from both the horizontally polarized and vertically polarized channels in current systems, a single-pol pulse is transmitted followed by a period of delay while reflected signals are received. The orthogonal pulse is subsequently sent and corresponding data is received during a second period of delay time. Therefore, reception of the reflected signals of both polarities occurs over these two delay periods, resulting in an increased overall dwell time. Similarly, because the dwell time is doubled, computational speed is effectively halved.
Other dual-pol radar systems, such as radars in the current NOAA weather radar network, simultaneously transmit the horizontal and vertical signals using, for example, crossed dipole antenna elements, to solve the issues of longer dwell times and velocity range reductions. While reducing these negative effects, these arrangements have problems with interference between the crossed horizontal and vertical signal components. Moreover, units having improved cross-pol isolation are costly, and thus unsuitable for use in the large antenna arrays necessary for meeting the narrow beamwidth requirements of some agencies.
Accordingly, an alternative means to implement dual-pol operation of a MPAR system with improved isolation is required.